The native snake venom disintegrin bitistatin binds with high affinity to the glycoprotein IIb/IIa receptor on activated platelets. When radiolabeled with 123I or 99mTc, bitistatin has been found to be significantly better than other tracers for rapid imaging of experimental pulmonary emboli (PE) and deep venous thrombi (DVT). The overall goal of this project is to develop an optimal platelet-targeting radiopharmaceutical for imaging PE and DVT based on bitistatin. Bitistatin will be produced by recombinant techniques. The characteristics of recombinant bitistatin (r-bitistatin) will be compared with native bitistatin with respect to its size, binding to platelets (both nonstimulated and stimulated), stability and distribution in whole blood. The ability of Tc-99m-r-bitistatin to image DVT and PE will then be evaluated in an animal model to document that image quality and tissue distribution of the Tc-99m labeled r- bitistatin is the same as native bitistatin. Recombinant bitistatin will then be produced in sufficient quantities for carrying out an initial clinical trial. Large scale modification of the r-bitistatin with bifunctional chelating agents to permit radiolabeling with 99mTc will be performed. The modified r- bitistatin will be tested for purity, safety, stability and biodistribution. Under an IND, an initial Phase I/II clinical trial will be conducted to assess the safety and efficacy of using 99mTc-bitistatin to image DVT in human subjects. At the same time, studies will continue on the evaluation of smaller peptide analogues of bitistatin which mimic the essential binding features of bitistatin. These peptides will be produced by chemical synthesis or by recombinant expression. The behavior of the labeled peptides will be assessed in vitro and compared with labeled native bitistatin. The ability of radiolabeled synthetic peptides to image DVT and PE will then be evaluated in a standardized animal model. Image quality and tissue distribution of the labeled peptides will be determined and compared with labeled native bitistatin. The study will identify one or more radiolabeled peptides which have potential clinical utility for rapid, direct imaging of DVT and PE as well as increase our understanding of the in vitro and in vivo characteristics that are important for imaging thrombotic lesions with platelet- directed radiotracers.